One well known way of etching a desired pattern or image onto a substrate involves first printing onto the substrate an ink, known as a resist ink, onto those parts of the substrate which are not to be etched to protect or mask those areas from the etching medium. The substrate carrying the resist ink mask is then exposed to the etching medium, for example, an aqueous acid, and the areas of the substrate which are not masked by the ink are etched. Following the etching step, the resist ink is removed from the substrate, for example, by washing in alkali. This process is widely used in the fabrication of printed circuit boards, where multiple deposition, masking, and etching steps are used to build up complex structures. Resist inks are also used in etching during fabrication of solar cells and steel embossing belts.
As well as etching, resist inks find application in electroplating. In that situation, the resist ink masks areas of the substrate from the electroplating medium and thereby prevents deposition on those areas. This process is used in the manufacture of some solar cells.
Screen printing is widely used to apply resist inks to the substrate. That technique is low cost and reliable.
The inks used in screen printing of resists are often photocurable, so that the ink can be cured to a hard film immediately after it has been printed onto the substrate. Such screens inks typically comprise up to 40% of an alkali soluble resin to impart alkali solubility to the cured ink layer so that they can be washed off the substrate with aqueous alkali.
Recently, attempts have been made to print resist images using ink jet printing. Ink jet printing has some advantages over screen printing, for example, images stored in computer readable form can be printed directly to the substrate and a higher resolution may be achieved. However, the adoption of ink jet as a method of printing resists has been limited, in part, because the currently available ink jet inks do not have the combination of properties including easy removeability desired in a resist ink.
In particular, because ink jet inks must have a very low viscosity in order to be jetted properly, the inks tend to spread rapidly on the surface of the substrate before they are cured, especially where the substrate has a non-flat or structured surface, thereby losing edge definition. Additionally, conventional ink jet inks are difficult to remove from the substrate, requiring the use of organic solvents and/or mechanical scraping or abrasion.
There remains a need for improved resist inks and for improved methods of etching and plating involving resist inks.